Anaerobic digester-membrane bioreactor for treating a waste stream

Abstract

A waste stream having anaerobically biodegradable components is fed to an anaerobic reactor where the components react with microorganisms to biodegrade the components and produce biomass and biogas. Mixing occurs in select portions of the anaerobic reactor, particularly the bottom and top portions of the reactor. Relatively heavy solids settle to the bottom and are mixed with the mixed liquor while relatively light or fine solids float to the top portion of the anaerobic reactor where they are mixed with the mixed liquor. This leaves an intermediate or middle portion of the anaerobic reactor where the concentration of solids is relatively lower compared to the concentration of solids in the upper or lower portion of the anaerobic reactor. Mixed liquor is pumped from the intermediate portion of the anaerobic reactor to a membrane separation unit where the mixed liquor is separated into a permeate stream and a retentate stream that is concentrated with solids. The retentate stream is recycled back to the anaerobic reactor and mixed with the mixed liquor therein. Further, mixed liquor and the relatively heavy solids are pumped from the bottom of the anaerobic reactor to a solids separator such as a hydrocyclone. Here the mixed liquor and solids are separated into a stream concentrated with heavier solids including a high percentage of inorganic precipitants and a lighter solids stream that contains a high percentage of biomass. This lighter solids liquid stream containing mixed liquor is recycled back to the anaerobic reactor for further treatment while the solid stream can be directed to a dewatering unit.

1 . A method of treating a waste stream having anaerobically biodegradable solids in an anaerobic membrane bioreactor, the method comprising:
 a. feeding the waste stream having the anaerobically biodegradable components into an anaerobic reactor;   b. reacting the biodegradable solids with anaerobic biomass in the anaerobic reactor to reduce the amount of biodegradable solids and components and in the process produce a biomass and biogas in the anaerobic reactor;   c. providing a mixing action in a lower portion of the anaerobic reactor and mixing the mixed liquor and relatively heavy solids in the bottom of the anaerobic reactor;   d. providing a mixing action in an upper portion of the anaerobic reactor and mixing the mixed liquor and relatively light solids in the upper portion of the anaerobic reactor;   e. wherein mixed liquor in an intermediate portion of the anaerobic reactor includes a relatively lower concentration of solids, including biomass;   f. pumping mixed liquor from the intermediate portion of the anaerobic reactor to a membrane separation unit and separating the mixed liquor into a permeate stream and a retentate stream concentrated with the solids;   g. recycling at least a portion of the retentate stream to the anaerobic reactor and mixing the retentate stream with mixed liquor in the anaerobic reactor;   h. pumping mixed liquor and at least some of the relatively heavy solids from the bottom portion of the anaerobic reactor to a solids separator and separating the heavier solids and the lighter solids containing biomass; and   i. recycling at least a portion of the lighter solids containing the biomass to the anaerobic reactor and mixing the lighter solids in the anaerobic reactor.   
     
     
         2 . The method of  claim 1  including directing the heavier solids from the solids separator to a dewatering unit and dewatering the heavier solids. 
     
     
         3 . The method of  claim 1  wherein there is one or more mixers disposed in the bottom portion of the anaerobic reactor and one or more mixers disposed in the upper portion of the anaerobic reactor. 
     
     
         4 . The method of  claim 3  wherein there are no mixers disposed in the intermediate portion of the reactor, and wherein the solids concentration of the mixed liquor in the intermediate portion of the reactor is less than the solids concentration in the mixed liquor in the upper portion or bottom portion of the anaerobic reactor such that fouling of membranes that form a part of the membrane separation unit is reduced, long term membrane performance is enhanced and membrane life is extended by minimizing the frequency of CIP cycles while maintaining a substantial concentration of mixed liquor volatile suspended solids inside the reactor. 
     
     
         5 . The method of  claim 1  including directing a contaminated stream to a membrane filtering unit and filtering the contaminated stream to produce a permeate stream and a second retentate stream, and wherein the permeate stream forms the waste stream that is treated by the anaerobic membrane bioreactor. 
     
     
         6 . An anaerobic membrane bioreactor for anaerobically biodegrading components in a waste stream, comprising:
 a. an anaerobic reactor having an inlet for permitting the waste stream having anaerobically biodegradable components to be directed into the anaerobic reactor resulting in the reaction of the biodegradable components with anaerobic biomass in the reactor to reduce the amount of biodegradable components and produce biomass and biogas in the anaerobic reactor;   b. one or more first mixers for providing a mixing action in a lower portion of the anaerobic reactor and mixing the mixed liquor and relatively heavy solids in the bottom of the anaerobic reactor;   c. one or more second mixers for providing a mixing action in an upper portion of the anaerobic reactor and mixing the mixed liquor and relatively light solids in the upper portion of the anaerobic reactor such that mixed liquor in an intermediate portion of the anaerobic reactor includes a relatively lower concentration of solids and biomass;   d. a first pump for pumping mixed liquor from the intermediate portion of the anaerobic reactor to a membrane separation unit and separating the mixed liquor into a permeate stream and a retentate stream concentrated with the biomass;   e. a first recycle line for recycling the retentate stream to the anaerobic reactor and mixing the retentate stream with mixed liquor in the anaerobic reactor;   f. a second pump for pumping mixed liquor and at least some of the relatively heavy solids from the bottom portion of the anaerobic reactor to a solids separator and separating the relatively heavy solids into two streams, a first stream including relatively heavy solids and a second stream including relatively lighter solids; and   a second recycle line for recycling at least a portion of the second stream to the anaerobic reactor and mixing the second stream with the mixed liquor in the anaerobic reactor.   
     
     
         7 . The anaerobic membrane bioreactor of  claim 6  wherein the anaerobic membrane bioreactor is operative to stratify the mixed liquor into at least three zones in the anaerobic reactor, a lower zone where the mixed liquor includes relatively heavy solids, an upper zone where the mixed liquor includes relatively light solids, and an intermediate zone in the anaerobic reactor between the lower and upper zones and where the mixed liquor includes a solids concentration that is measurably less than the solids concentration in the mixed liquor in the lower zone. 
     
     
         8 . The anaerobic membrane bioreactor of  claim 7  including a mixed liquor feed line for directing mixed liquor from the anaerobic reactor to the membrane separation unit, and wherein the mixed liquor feed line is plumbed into the anaerobic reactor such that mixed liquor from the intermediate zone, as opposed to the lower and upper zones, is pumped into the mixed liquor feed line and directed to the membrane separation unit. 
     
     
         9 . The anaerobic membrane bioreactor of  claim 6  further including:
 a. a mixed liquor feed line connected between the anaerobic reactor and the membrane separation unit for directing mixed liquor from the anaerobic reactor to the membrane separation unit; 
 b. the mixed liquor feed line connected to the anaerobic reactor such that most of the mixed liquor directed to the membrane separation unit comes from the intermediate portion of the anaerobic reactor; 
 c. a mixed liquor and solids feed line operatively connected between the anaerobic reactor and the solids separator; and 
 d. the mixed liquor and solids feed line connected to the lower portion of the anaerobic reactor such that the mixed liquor and solids fed to the solids separator comes from the lower portion of the anaerobic reactor. 
 
     
     
         10 . A method of treating a waste stream having anaerobically biodegradable components, comprising:
 a. feeding the waste stream having the anaerobically biodegradable components into an anaerobic reactor;   b. reacting the biodegradable components with anaerobic biomass in the anaerobic reactor to reduce the amount of biodegradable components and in the process produce biomass and biogas;   c. stratifying the mixed liquor in the anaerobic reactor by forming a first lower mixed liquor zone where the mixed liquor in the first lower mixed liquor zone includes a relatively high concentration of solids, and forming a second mixed liquor zone above the first lower mixed liquor zone where the mixed liquor in the second zone includes a solids concentration substantially less than the concentration of solids in the mixed liquor in the first lower mixed liquor zone;   d. directing mixed liquor from the second mixed liquor zone in the anaerobic reactor to a membrane separation unit and separating the mixed liquor into a permeate stream and a retentate stream concentrated with biomass;   e. recycling at least a portion of the retentate stream to the anaerobic reactor and mixing the retentate stream with mixed liquor in the reactor;   f. directing the mixed liquor and solids from the first lower mixed liquor zone to a solids separator and separating the mixed liquor and solids from the first lower mixed liquor zone into a heavier solids stream and a lighter solids stream containing biomass; and   g. recycling at least a portion of the lighter solids stream containing biomass from the solids separator to the anaerobic reactor and mixing the lighter solids stream with the mixed liquor in the anaerobic reactor.   
     
     
         11 . The method of  claim 10  including providing a mixing action in the first lower mixed liquor zone and mixing the mixed liquor and solids therein; and maintaining the mixed liquor in the second zone in an unmixed state or in a state where the mixing action in the second zone is substantially less than the mixing action in the first lower mixed liquor zone. 
     
     
         12 . The method of  claim 10  wherein the method includes forming a third mixed liquor zone over the second mixed liquor zone; and wherein both the first and third mixed liquor zones are mixed with relatively heavy solids residing in the first mixed liquor zone and relatively light solids residing in the third mixed liquor zone. 
     
     
         13 . The method of  claim 10  wherein the solids separator includes a hydrocyclone. 
     
     
         14 . The method of  claim 10  wherein during certain time intervals both the membrane separation unit and the solids separator are operated simultaneously and wherein there is one mixed liquor flow from the anaerobic reactor to the membrane separation unit and another mixed liquor flow from the anaerobic reactor to the solids separator, and wherein the two flows are independent of each other. 
     
     
         15 . The method of  claim 12  wherein the second mixed liquor zone is unmixed and wherein the concentration of solids within the second mixed liquor zone is less than the concentration of solids in the first or third mixed liquor zones. 
     
     
         16 . A method of treating a waste stream having anaerobically biodegradable components, comprising:
 a. feeding a waste stream having the anaerobically biodegradable components into an anaerobic reactor;   b. reacting the biodegradable components with anaerobic biomass in the anaerobic reactor to reduce the amount of biodegradable components and in the process produce biomass and biogas;   c. stratifying the mixed liquor in the anaerobic reactor into three mixed liquor zones by forming a first lower mixed liquor zone where the mixed liquor in the first lower mixed liquor zone includes relatively heavy solids, forming a second mixed liquor zone above the first lower mixed liquor zone where the mixed liquor in the second mixed liquor zone includes a solids concentration substantially less than the concentration of solids in the first lower mixed liquor zone, and forming a third mixed liquor zone over the second mixed liquor zone where the third mixed liquor zone includes relatively light solids;   d. directing mixed liquor from the second mixed liquor zone in the anaerobic reactor to a membrane separation unit and separating the mixed liquor into a permeate stream and a retentate stream; and   e. recycling at least a portion of the retentate stream to the anaerobic reactor and mixing the retentate stream with the mixed liquor in the reactor.   
     
     
         17 . The method of  claim 16  including directing the mixed liquor from the first lower mixed liquor zone to a solids separator and separating the mixed liquor from the first lower mixed liquor zone into a heavier solids stream and a lighter solids stream containing biomass. 
     
     
         18 . The method of  claim 16  including mixing the mixed liquor in the first lower mixed liquor zone and mixing the mixed liquor in the third mixed liquor zone. 
     
     
         19 . The method of  claim 18  wherein the mixed liquor in the second mixed liquor zone is generally unmixed such that the mixing action in the first and third mixed liquor zones is greater than the mixing action in the second mixed liquor zone. 
     
     
         20 . The method of  claim 19  including providing one or more mixers in a lower portion of the anaerobic reactor for mixing the mixed liquor in the first lower mixed liquor zone; and providing one or more mixers in an upper portion of the anaerobic reactor for mixing the mixed liquor in the third mixed liquor zone. 
     
     
         21 . The method of  claim 10  wherein the heavier solids stream contains a relatively high percentage of inorganic precipitants compared to the lighter solids stream. 
     
     
         22 . A method of treating a stream containing soluble and insoluble COD in an anaerobic membrane bioreactor, the method comprising:
 a. feeding the stream having soluble and insoluble COD into an anaerobic reactor;   b. reacting the soluble and insoluble COD with anaerobic biomass in the anaerobic reactor to reduce the amount of soluble and insoluble COD and in the process produce a mixed liquor and biogas in the anaerobic reactor;   c. providing a mixing action in a lower portion of the anaerobic reactor to mix the relatively heavy solids in the mixed liquor;   d. providing a mixing action in an upper portion of the anaerobic reactor to mix the relatively light solids in the mixed liquor:   e. wherein in an intermediate portion of the anaerobic reactor a relatively lower concentration of solids in mixed liquor is present;   f. pumping the mixed liquor from the intermediate portion of the anaerobic reactor to a membrane separation unit and separating the mixed liquor into a permeate stream and a retentate stream which substantially contains the concentrated biomass;   g. recycling at least a portion of the retentate stream to the anaerobic reactor and;   h. pumping mixed liquor with the relatively heavy solids from the bottom portion of the anaerobic reactor to a solids separator, and separating the heavier solids from the lighter solids containing biomass; and   i. recycling at least a portion of the lighter solids containing the biomass to the anaerobic reactor.